That Butler, Friendly is Chapter 96.5 of the Kuroshitsuji manga. The extra chapter is a side story to the Circus Arc.
Summary[]
During the first night after Ciel Phantomhive entered the Noah's Ark Circus, Freckles asks if he is hungry, and drags him to eat at the mess hall. They sit with Joker, Peter, and Dagger. Sebastian Michaelis serves them food, and is offered a drink. Dagger tries to get Ciel to drink as well, but Peter warns that his teeth will go bad if he starts drinking at a young age. When Freckles mentions the Tooth Fairy, Joker recites the legend: if a tooth that falls out has no cavities, then it will be traded with a coin; he concedes that it is a made-up story designed to get children to brush their teeth and that the coin is usually provided by the parents. He says that back then, Doll was not aware of that.
Ten years ago, in East End, Doll excitedly told Joker, Dagger, Beast, Peter, Wendy, and Jumbo that her tooth had fallen out, and so, she will use the coin she is supposed to get to buy them bread. Peter was about to expose to her that the Tooth Fairy is not real, but Jumbo stopped him. Doll eagerly headed off to broadcast the news to other people. Jumbo admitted that Doll looked so happy that he could not bear to see her disillusioned. Beast commented that Doll will probably be very upset if she gets no coin the next day. Joker suggested earning one, but Peter pointed out the impossibility of getting a job and stated that they must steal like they always do. Joker said he does not want to steal this time, as it would not feel right, resulting in a frustrated Peter, who exclaimed that he hates kids. He, then, told them to hurry and get going, declaring that they will cooperate and accomplish the task together, which made the rest of them smile.
They then searched, begged, and labored strenuously. The following morning, they, though exhausted and sleep-deprived, had finally attained a coin. Doll was ecstatic upon finding the coin when she woke up, and she enthusiastically proclaimed that they could now eat bread. They all rejoiced.
Joker finishes his story. Peter comments that it got problematic after Doll tried to take out teeth that were not even wobbly. Sebastian then shares his story about Ciel.
Two years ago, an eleven years old Ciel informed Sebastian that one of his teeth was wobbly, and Sebastian examined it. When Sebastian announced that he will remove it momentarily, a fearful Ciel attempted to resist, but the tooth was pulled out forcefully anyway.
The troupe members are speechless, as Sebastian confesses that he thought he was doing a favor, but Ciel, with a bloody mouth, had thrown a fit afterward. He adds that, from then on, Ciel never allowed him to check his teeth for cavities. When Sebastian claims it was a lot of trouble for himself, Dagger argues that it was Ciel who experienced the most pain; Sebastian lightheartedly responds that he had no experience with taking care of children back then.
Subsequently, Sebastian says that it must be very inconvenient for humans to not be able to grow back teeth as soon as they come out. Freckles whispers in Ciel's ear that Sebastian is rather strange, and Ciel agrees.